Thermochromatic device and thermochromatic display apparatus

ABSTRACT

A thermochromatic device includes an insulating substrate, a color element, a heating element, a first electrode, and a second electrode. The color element is located on the insulating substrate and includes a reversible thermochromatic material. The heating element is located adjacent to the color element and includes a carbon nanotube structure. The first electrode and the second electrode are electrically connected to the heating element. A thermochromatic display apparatus using the thermochromatic device is also related.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 200910189148.3, filed on Dec. 18, 2009 inthe China Intellectual Property Office, disclosure of which isincorporated herein by reference. This application is related toapplications entitled, “THERMOCHROMATIC DEVICE AND THERMOCHROMATICDISPLAY APPARATUS”, filed Sep. 17, 2010 Ser. No. 12/884,627; and“THERMOCHROMATIC DEVICE AND THERMOCHROMATIC DISPLAY APPARATUS”, filedSep. 17, 2010 Ser. No. 12/884,641.

BACKGROUND

1. Technical Field

The present disclosure relates to a thermochromatic device andthermochromatic display apparatus using the same.

2. Description of Related Art

Thermochromatic materials are materials that can change their color inresponse to changes in temperature. Thermochromatic materials can beused to make a thermochromatic device.

A thermochromatic device, according to the prior art usually includes asupport substrate, a thermochromatic material layer located on a surfaceof the support substrate, and a heater. The heater is used to heat thethermochromatic material layer. The heater is usually made of ceramics,conductive glasses or metals. However, a color change speed of thethermochromatic device is slow because the relatively high heat capacityper unit and slow heating speed of the heater.

What is needed, therefore, is to provide a thermochromatic device havingan improved color change speed and thermochromatic display apparatususing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a schematic view of one embodiment of a thermochromaticdevice.

FIG. 2 is a Scanning Electron Microscope (SEM) image of a drawn carbonnanotube film.

FIG. 3 is a schematic of a carbon nanotube segment.

FIG. 4 is an SEM image of an untwisted carbon nanotube wire.

FIG. 5 is an SEM image of a twisted carbon nanotube wire.

FIG. 6 is a schematic view of one embodiment of a thermochromaticdevice.

FIG. 7 is a schematic view of one embodiment of a thermochromaticdevice.

FIG. 8 is a schematic view of one embodiment of a thermochromaticdevice.

FIG. 9 is a schematic view of one embodiment of a thermochromaticdevice.

FIG. 10 is a schematic view of one embodiment of a thermochromaticdevice.

FIG. 11 is a schematic view of a thermochromatic display apparatus usingthe thermochromatic device of FIG. 1.

FIG. 12 is a schematic, cross-sectional view, along a line XII-XII ofFIG. 11.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

References will now be made to the drawings to describe, in detail,various embodiments of the present thermochromatic device andthermochromatic display apparatus using the same.

Referring to FIG. 1, a thermochromatic device 220 of a first embodimentincludes an insulating substrate 202, a color element 218, a heatingelement 208, a first electrode 210, and a second electrode 212.

The insulating substrate 202 has a top surface (not labeled). The colorelement 218, the heating element 208, the first electrode 210 and thesecond electrode 212 are located on the top surface of the insulatingsubstrate 202. The heating element 208 is located adjacent to the colorelement 218 and configured to heat the color element 218. The heatingelement 208 can be located substantially adjacent to, about, above, on,or under the color element 218. Any arrangement can be made just as longas the heating element 208 can heat the color element 218. The firstelectrode 210 and the second electrode 212 are located apart from eachother and electrically connected to the heating element 208. In oneembodiment, both the heating element 208 and the color element 218 arelayered structures. The heating element 208 is located on the surface ofthe insulating substrate 202. The first electrode 210 and the secondelectrode 212 are located respectively on a surface of the heatingelement 208 and at the two opposite sides of the heating element 208.The color element 218 is located on a surface of the heating element 208and between the first electrode 210 and the second electrode 212.

The insulating substrate 202 may be made of rigid material or flexiblematerial. The rigid material may be ceramic, glass, quartz, resin,silicon, silicon dioxide, diamond, or alumina. The flexible material maybe flexible polymer, fiber, or synthetic paper. The flexible polymer canbe polyethylene terephthalate (PET), polycarbonate (PC), polyethylene(PE), or polyimide (PI). When the insulating substrate 202 is made offlexible material, the thermochromatic device 220 can be folded intorandom shapes during use. The melting point of the insulating substrate202 is equal to or higher than 200° C. A size and a thickness of theinsulating substrate 202 can be chosen according to need. In oneembodiment, the insulating substrate 202 is a PET film with a thicknessof about 1 millimeter.

The color element 218 is made of reversible thermochromatic material.The color of the reversible thermochromatic material may change with thechange of the environmental temperature and will come back to originalcolor as the environmental temperature comes back to it's originaltemperature. The color change temperature of the reversiblethermochromatic material is below 200° C. in one embodiment. In oneembodiment, the color change temperature of the reversiblethermochromatic material is in a range from about 40° C. to about 100°C. so that the thermochromatic device 220 can work at room temperatureand have a low working voltage. The reversible thermochromatic materialcan be inorganic reversible thermochromatic material, organic reversiblethermochromatic material, or liquid crystal reversible thermochromaticmaterial.

Inorganic reversible thermochromatic materials include silver iodide,silver complex, silver double salt, copper iodide, copper complex,copper double salt, mercury iodide, mercury complex, mercury doublesalt, cobalt salt, nickel salt, methenamine compound, vanadium (III)oxide, chromate, or vanadate. Some inorganic reversible thermochromaticmaterials and their color change range, and color change temperature isshow in table 1 below.

TABLE 1 Inorganic Reversible Thermochromatic Material Color ChangeMaterial (Chemical Formula) Color Change Range TemperatureCoCl₂•2C₆H₁₂N₄•10H₂O Pink-Sky Blue 39.6° C.   CoI₂•2C₆H₁₂N₄•10H₂OPink-Green 50° C. CoSO₄•C₆H₁₂N₄•9H₂O Peachblow-Purple 60° C.CuSO₄•2C₆H₁₂N₄•5H₂O Blue-Emerald 76° C. NiBr₂•2C₆H₁₂N₄•10H₂O Green-Blue60° C. NiC₁₂•2C₆H₁₂N₄•10H₂O Green-Yellow 110° C. Co(NO₃)₂•2C₆H₁₂N₄•10H₂O Pink-Crimson 75° C. Ag₂HgI₄ Yellow-Red 42° C.Cu₂HgI₄ carmine-Brick Red 71° C. HgI₂ Red-Blue 137° C. 

The organic reversible thermochromatic material includes color fixatives(electron donor), color developing agent (electron acceptor), andsolvent. The color of the organic reversible thermochromatic materialdepends on the color fixatives. The color depth of the organicreversible thermochromatic material depends on the color developingagent. The color change temperature of the organic reversiblethermochromatic material depends on the solvent. The color fixatives canbe triarylmethane dyes, fluorane dyes, thiodiphenylamine, spiropyrandyes, Schiff-base dyes, spiro compounds, bianthrone, or combinationthereof. The triarylmethane dyes can be Crystal violet lactone, orcresol red. The fluorane dyes can be thermochromic red or thermochromicgreen. The color developing agent includes organic color developingagent or inorganic color developing agent. The inorganic colordeveloping agent can be acid clay, activated clay or kaoline, ormagnesium aluminum silicate. The organic color developing agent can bebisphnol A, benzyl hydroxybenzoate, 4-hydroxycoumarin, n-hexanoic acid,caprylic acid, stearic acid, terephthalic acid, or Lewis acid. Thesolvent can be dodecanol, n-tetradecyl alcohol, hexadecanol, n-octadecylalcohol, aliphatic ketones, ester, aether, amides, or carboxylic acidcompound.

The liquid crystal reversible thermochromatic material can bethermotropic liquid crystals. The thermotropic liquid crystals consistof organic molecules and exhibit a phase transition into the liquidcrystal phase as temperature is changed. The thermotropic liquidcrystals can be divided into nematic liquid crystals, smectic liquidcrystals, or cholesteric liquid crystals according to its opticalproperty. The cholesteric liquid crystals can be made of cholesterin.

In one embodiment, the color element 218 is a layer of Ag₂HgI₄ with athickness from about 10 micrometers to about 500 micrometers. In otherembodiments, the color element 218 is a layer of Ag₂HgI₄ with athickness from about 50 micrometers to about 100 micrometers. The colorelement 218 can be formed by sputtering or thermal deposition. The colorelement 218 is located between the first electrode 210 and the secondelectrode 212. The color element 218 can be spaced from the firstelectrode 210 and the second electrode 212 or make contact with thefirst electrode 210 and the second electrode 212.

The heating element 208 includes a carbon nanotube structure. The carbonnanotube structure includes a plurality of carbon nanotubes uniformlydistributed therein, and the carbon nanotubes therein can be combined byvan der Waals attractive force therebetween. The carbon nanotubestructure can be a substantially pure structure of the carbon nanotubes,with few impurities. The carbon nanotubes can be used to form manydifferent structures and provide a large specific surface area. The heatcapacity per unit area of the carbon nanotube structure can be less than2×10⁻⁴ J/m²·K. In one embodiment, the heat capacity per unit area of thecarbon nanotube structure is less than 1.7×10⁻⁶ J/m²·K. As the heatcapacity of the carbon nanotube structure is very low, and thetemperature of the heating element 208 can rise and fall quickly, whichmakes the heating element 208 have a high heating efficiency andaccuracy. As the carbon nanotube structure can be substantially pure,the carbon nanotubes are not easily oxidized and the life of the heatingelement 208 will be relatively long. Further, the carbon nanotubes havea low density, about 1.35 g/cm³, so the heating element 208 is light. Asthe heat capacity of the carbon nanotube structure is very low, theheating element 208 has a high response heating speed. As the carbonnanotube has large specific surface area, the carbon nanotube structurewith a plurality of carbon nanotubes has large specific surface area.When the specific surface of the carbon nanotube structure is largeenough, the carbon nanotube structure is adhesive and can be directlyapplied to a surface.

The carbon nanotubes in the carbon nanotube structure can be arrangedorderly or disorderly. The term ‘disordered carbon nanotube structure’includes, but is not limited to, to a structure where the carbonnanotubes are arranged along many different directions, and the aligningdirections of the carbon nanotubes are random. The number of the carbonnanotubes arranged along each different direction can be almost the same(e.g. uniformly disordered). The disordered carbon nanotube structurecan be isotropic. The carbon nanotubes in the disordered carbon nanotubestructure can be entangled with each other.

The carbon nanotube structure including ordered carbon nanotubes is anordered carbon nanotube structure. The term ‘ordered carbon nanotubestructure’ includes, but is not limited to, to a structure where thecarbon nanotubes are arranged in a consistently systematic manner, e.g.,the carbon nanotubes are arranged approximately along a same directionand/or have two or more sections within each of which the carbonnanotubes are arranged approximately along a same direction (differentsections can have different directions). The carbon nanotubes in thecarbon nanotube structure can be selected from a group consisting ofsingle-walled, double-walled, and/or multi-walled carbon nanotubes.

The carbon nanotube structure can be a carbon nanotube film structurewith a thickness ranging from about 0.5 nanometers to about 1millimeter. The carbon nanotube film structure can include at least onecarbon nanotube film. The carbon nanotube structure can also be a linearcarbon nanotube structure with a diameter ranging from about 0.5nanometers to about 1 millimeter. The carbon nanotube structure can alsobe a combination of the carbon nanotube film structure and the linearcarbon nanotube structure. It is understood that any carbon nanotubestructure described can be used with all embodiments. It is alsounderstood that any carbon nanotube structure may or may not employ theuse of a support structure.

When the heating element 208 is a carbon nanotube film structure, thecarbon nanotube film structure can be located on the surface of theinsulating substrate 202 or the surface of the color element 218. Whenthe heating element 208 is a linear carbon nanotube structure, thelinear carbon nanotube structure can be located around the color element218.

In one embodiment, the carbon nanotube film structure includes at leastone drawn carbon nanotube film. A drawn carbon nanotube film can bedrawn from a carbon nanotube array that is able to have a film drawntherefrom. The drawn carbon nanotube film includes a plurality ofsuccessive and oriented carbon nanotubes joined end-to-end by van derWaals attractive force therebetween. The drawn carbon nanotube film is afree-standing film. Referring to FIGS. 2 to 3, each drawn carbonnanotube film includes a plurality of successively oriented carbonnanotube segments 143 joined end-to-end by van der Waals attractiveforce therebetween. Each carbon nanotube segment 143 includes aplurality of carbon nanotubes 145 parallel to each other, and combinedby van der Waals attractive force therebetween. As can be seen in FIG.2, some variations can occur in the drawn carbon nanotube film. Thecarbon nanotubes 145 in the drawn carbon nanotube film are orientedalong a preferred orientation. The carbon nanotube film can be treatedwith an organic solvent to increase the mechanical strength andtoughness and reduce the coefficient of friction of the carbon nanotubefilm. A thickness of the carbon nanotube film can range from about 0.5nanometers to about 100 micrometers. In one embodiment, the heatingelement 208 is a single drawn carbon nanotube film with a length of 300micrometers and a width of 100 micrometers. The carbon nanotubes of theheating element 208 extend from the first electrode 210 to the secondelectrode 212. The drawn carbon nanotube film can be attached tosurfaces of the insulating substrate 202 with an adhesive, by mechanicalforce, by the adhesive properties of the carbon nanotube film, or by acombination thereof. The response speed of the drawn carbon nanotubefilm is very high because of the very low heat capacity per unit area,the large surface area, and the large radiation coefficient. Thetemperature of the single drawn carbon nanotube film with a length of300 micrometers and a width of 100 micrometers can be risen to 2000Kelvin in 1 millisecond.

The carbon nanotube film structure of the heating element 208 caninclude at least two stacked drawn carbon nanotube films. In otherembodiments, the carbon nanotube structure can include two or morecoplanar carbon nanotube films, and can include layers of coplanarcarbon nanotube films. Additionally, when the carbon nanotubes in thecarbon nanotube film are aligned along one preferred orientation (e.g.,the drawn carbon nanotube film), an angle can exist between theorientation of carbon nanotubes in adjacent films, whether stacked oradjacent. Adjacent carbon nanotube films can be combined by only the vander Waals attractive force therebetween. The number of the layers of thecarbon nanotube films is not limited as long as the carbon nanotubestructure. However the thicker the carbon nanotube structure, thespecific surface area will decrease. An angle between the aligneddirections of the carbon nanotubes in two adjacent carbon nanotube filmscan range from about 0° to about 90°. When the angle between the aligneddirections of the carbon nanotubes in adjacent stacked carbon nanotubefilms is larger than 0 degrees, a microporous structure is defined bythe carbon nanotubes in the heating element 208. The carbon nanotubestructure in an embodiment employing these films will have a pluralityof micropores. Stacking the carbon nanotube films will also add to thestructural integrity of the carbon nanotube structure. In someembodiments, the carbon nanotube structure is a free standing structure.

In another embodiment, the carbon nanotube film structure includes aflocculated carbon nanotube film. The flocculated carbon nanotube filmcan include a plurality of long, curved, disordered carbon nanotubesentangled with each other. Further, the flocculated carbon nanotube filmcan be isotropic. The carbon nanotubes can be substantially uniformlydispersed in the carbon nanotube film. Adjacent carbon nanotubes areacted upon by van der Waals attractive force to form an entangledstructure with micropores defined therein. It is understood that theflocculated carbon nanotube film is very porous. Sizes of the microporescan be less than 10 micrometers. The porous nature of the flocculatedcarbon nanotube film will increase specific surface area of the carbonnanotube structure. Further, due to the carbon nanotubes in the carbonnanotube structure being entangled with each other, the carbon nanotubestructure employing the flocculated carbon nanotube film has excellentdurability, and can be fashioned into desired shapes with a low risk tothe integrity of the carbon nanotube structure. The flocculated carbonnanotube film, in some embodiments, will not require the use of theplanar supporter 18 due to the carbon nanotubes being entangled andadhered together by van der Waals attractive force therebetween. Thethickness of the flocculated carbon nanotube film can range from about0.5 nanometers to about 1 millimeter.

In another embodiment, the carbon nanotube film structure can include atleast a pressed carbon nanotube film. The pressed carbon nanotube filmcan be a free-standing carbon nanotube film. The carbon nanotubes in thepressed carbon nanotube film are arranged along a same direction orarranged along different directions. The carbon nanotubes in the pressedcarbon nanotube film can rest upon each other. Adjacent carbon nanotubesare attracted to each other and combined by van der Waals attractiveforce. An angle between a primary alignment direction of the carbonnanotubes and a surface of the pressed carbon nanotube film is 0 degreesto approximately 15 degrees. The greater the pressure applied, thesmaller the angle formed. When the carbon nanotubes in the pressedcarbon nanotube film are arranged along different directions, the carbonnanotube structure can be isotropic. The thickness of the pressed carbonnanotube film ranges from about 0.5 nanometers to about 1 millimeter.

Carbon nanotube structures include linear carbon nanotubes. In otherembodiments, the linear carbon nanotube structures, including carbonnanotube wires and/or carbon nanotube cables, can be used.

The carbon nanotube wire can be untwisted or twisted. Treating the drawncarbon nanotube film with a volatile organic solvent can form theuntwisted carbon nanotube wire. Specifically, the organic solvent isapplied to soak the entire surface of the drawn carbon nanotube film.During the soaking, adjacent parallel carbon nanotubes in the drawncarbon nanotube film will bundle together, due to the surface tension ofthe organic solvent as it volatilizes, and thus, the drawn carbonnanotube film will be shrunk into untwisted carbon nanotube wire.Referring to FIG. 4, the untwisted carbon nanotube wire includes aplurality of carbon nanotubes substantially oriented along a samedirection (i.e., a direction along the length of the untwisted carbonnanotube wire). The carbon nanotubes are parallel to the axis of theuntwisted carbon nanotube wire. More specifically, the untwisted carbonnanotube wire includes a plurality of successive carbon nanotubesegments joined end to end by van der Waals attractive forcetherebetween. Each carbon nanotube segment includes a plurality ofcarbon nanotubes substantially parallel to each other, and combined byvan der Waals attractive force therebetween. The carbon nanotubesegments can vary in width, thickness, uniformity and shape. Length ofthe untwisted carbon nanotube wire can be arbitrarily set as desired. Adiameter of the untwisted carbon nanotube wire ranges from about 0.5nanometers to about 100 micrometers.

The twisted carbon nanotube wire can be formed by twisting a drawncarbon nanotube film using a mechanical force to turn the two ends ofthe drawn carbon nanotube film in opposite directions. Referring to FIG.5, the twisted carbon nanotube wire includes a plurality of carbonnanotubes helically oriented around an axial direction of the twistedcarbon nanotube wire. More specifically, the twisted carbon nanotubewire includes a plurality of successive carbon nanotube segments joinedend to end by van der Waals attractive force therebetween. Each carbonnanotube segment includes a plurality of carbon nanotubes parallel toeach other, and combined by van der Waals attractive force therebetween.Length of the carbon nanotube wire can be set as desired. A diameter ofthe twisted carbon nanotube wire can be from about 0.5 nanometers toabout 100 micrometers. Further, the twisted carbon nanotube wire can betreated with a volatile organic solvent after being twisted. After beingsoaked by the organic solvent, the adjacent paralleled carbon nanotubesin the twisted carbon nanotube wire will bundle together, due to thesurface tension of the organic solvent when the organic solventvolatilizing. The specific surface area of the twisted carbon nanotubewire will decrease, while the density and strength of the twisted carbonnanotube wire will be increased.

The carbon nanotube cable includes two or more carbon nanotube wires.The carbon nanotube wires in the carbon nanotube cable can be, twistedor untwisted. In an untwisted carbon nanotube cable, the carbon nanotubewires are parallel with each other. In a twisted carbon nanotube cable,the carbon nanotube wires are twisted with each other.

The heating element 208 can include one or more linear carbon nanotubestructures. The plurality of linear carbon nanotube structures can beparalleled with each other, cross with each other, weaved together, ortwisted with each other. The resulting structure can be a planarstructure if so desired.

In other embodiments, the carbon nanotube structure can be a carbonnanotube layer formed by printing. The carbon nanotube layer includes aplurality of carbon nanotubes disorderly distributed therein.

In other embodiments, the carbon nanotube structure can include othermaterials thus becoming carbon nanotube composite. The carbon nanotubecomposite can include a carbon nanotube structure and a plurality offillers dispersed therein. The filler can be comprised of a materialselected from a group consisting of metal, ceramic, glass, carbon fiberand combinations thereof. Alternatively, the carbon nanotube compositecan include a matrix and a plurality of carbon nanotubes dispersedtherein. The matrix can be comprised of a material selected from a groupconsisting of resin, metal, ceramic, glass, carbon fiber andcombinations thereof. In one embodiment, a carbon nanotube structure ispackaged in a resin matrix.

The first electrode 210 and the second electrode 212 can be located onthe surface of the insulating substrate 202, on the surface of the colorelement 218, or on the surface of the heating element 208. The firstelectrode 210 and the second electrode 212 can be made of conductivematerial such as carbon nanotube, metal, alloy, indium tin oxides (ITO),antimony doped Tin oxide (ATO), conductive polymer, or a conductiveslurry. In one embodiment, the first electrode 210 and the secondelectrode 212 are formed on the surface of the heating element 208 by aprinting process. The conductive slurry is composed of metal powder,glass powder, and binder. The metal powder can be silver powder, theglass powder has low melting point, and the binder can be terpineol orethyl cellulose (EC). The conductive slurry can include from about 50%to about 90% (by weight) of the metal powder, from about 2% to about 10%(by weight) of the glass powder, and from about 8% to about 40% (byweight) of the binder.

Thermochromatic device 220 can be made by the following steps:

-   -   (a) laying a single drawn carbon nanotube film on a surface of        the insulating substrate 202 as a heating element 208;    -   (b) applying a first electrode 210 and a second electrode 212 on        the surface of the heating element 208; and    -   (c) depositing a layer of Ag₂HgI₄ between the first electrode        210 and the second electrode 212 as a color element 218.

During operation, a voltage is supplied to the first electrode 210 andthe second electrode 212. The temperature of the heating element 208raises and the color element 218 is heated by the heating element 208.When the color element 218 is heated to a color change temperature, thecolor of the color element 218 will change. For example, the colorelement 218 made of Ag₂HgI₄ will change color from yellow to red when itis heated to a color change temperature of 42° C. Supplying a constantvoltage, the temperature of the color element 218 will remain constant.Therefore, the thermochromatic device 220 will display a constant color.The color displayed by the thermochromatic device 220 can be changedthrough changing of the voltage being supplied, and it will change theenvironmental temperature of the color element 218. Because the colorelement 218 is made of reversible thermochromatic material, the color ofthe color element 218 will come back to original color as soon as itstops heating. Thus, the display reverts and the color is erased.

Referring to FIG. 6, a thermochromatic device 320 of one embodimentincludes an insulating substrate 302, a color element 318, a heatingelement 308, a first electrode 310, and a second electrode 312. Thethermochromatic device 320 is similar to the thermochromatic device 220described above except that the color element 318 is located between theinsulating substrate 302 and the heating element 308. In one embodiment,both the heating element 308 and the color element 318 are layeredstructures. The first electrode 310 and the second electrode 312 arelocated on a surface of the insulating substrate 302 and spaced fromeach other. The color element 318 is located on the surface of theinsulating substrate 302 and between the first electrode 310 and thesecond electrode 312. The heating element 308 is located on a surface ofthe color element 318 and covers the color element 318, the firstelectrode 310, and the second electrode 312. The heating element 308 isa single drawn carbon nanotube film, which is transparent.

Thermochromatic device 320 can be made by the following steps:

-   -   (d) applying a first electrode 310 and a second electrode 312 on        the surface of the insulating substrate 302;    -   (e) depositing a layer of Ag₂HgI₄ between the first electrode        310 and the second electrode 312 as a color element 318; and    -   (f) placing a single drawn carbon nanotube film on a surface of        the color element 318 to cover the color element 318, the first        electrode 310 and the second electrode 312.

Referring to FIG. 7, a thermochromatic device 420 of one embodimentincludes an insulating substrate 402, a color element 418, a heatingelement 408, a first electrode 410, and a second electrode 412. Thethermochromatic device 420 is similar to the thermochromatic device 320described above except that the heating element 408 is spaced from thecolor element 418. In one embodiment, both the heating element 408 andthe color element 418 are layered structures. The first electrode 410and the second electrode 412 are located on a surface of the insulatingsubstrate 402 and spaced from each other. The color element 418 islocated on the surface of the insulating substrate 402 and between thefirst electrode 410 and the second electrode 412. The color element 418is thinner than the electrodes 410 and 412. The heating element 408 islocated on a surface of the first electrode 410 and the second electrode412 and spaced from the color element 418. The heating element 408 is asingle drawn carbon nanotube film, which is a transparent and freestanding. The heat capacity per unit area of the single drawn carbonnanotube film is less than 1.7×10⁻⁶ J/m²·K. The main heat exchangemanner between the color element 418 and the heating element 408 is heatradiation. Because the single drawn carbon nanotube film has small heatcapacity per unit area, the heating element 408 can be heated to a hightemperature in short time and supply a short and intensive heat writepulse to the color element 418. Therefore, the thermochromatic device420 has an improved response speed.

Referring to FIG. 8, a thermochromatic device 520 of one embodimentincludes an insulating substrate 502, a color element 518, a heatingelement 508, a first electrode 510, and a second electrode 512. Thethermochromatic device 520 is similar to the thermochromatic device 220described above except that the heating element 508 is located betweenthe insulating substrate 502 and the color element 518 and extends to aprofile of the color element 518. In one embodiment, the first electrode510 and the second electrode 512 are located on a surface of theinsulating substrate 502 and spaced from each other. The color element518 is located between the first electrode 510 and the second electrode512. The heating element 508 is located between the insulating substrate502 and the color element 518 and extends to a top surface of theelectrodes 510 and 512 through a profile of the electrodes 510 and 512.The heating element 508 can also extend to a top surface of the colorelement 518 to package the color element 518. The heating element 508 isa single drawn carbon nanotube film, which is transparent and freestanding. Because the heating element 508 and the color element 518 havea larger contacting surface, the color element 518 can be heatedeffectively. Therefore, the thermochromatic device 520 has an improvedresponse speed.

Thermochromatic device 520 can be made by the following steps:

-   -   (g) applying a first electrode 510 and a second electrode 512 on        a surface of the insulating substrate 502;    -   (h) laying a single drawn carbon nanotube film on a surface of        the electrodes 510 and 512 to cover the first electrode 510 and        the second electrode 512;    -   (i) pressing the carbon nanotube film between the first        electrode 510 and the second electrode 512 so that the carbon        nanotube film attaches on the surface of the insulating        substrate 502 and two opposite surfaces of the electrodes 510        and 512; and    -   (j) depositing a layer of Ag₂HgI₄ between the first electrode        510 and the second electrode 512 as a color element 518.

Referring to FIG. 9, a thermochromatic device 620 of one embodimentincludes an insulating substrate 602, a color element 618, a firstheating element 608, a second heating element 609, a first electrode 610and a second electrode 612. The thermochromatic device 620 is similar tothe thermochromatic device 220 described above except that thethermochromatic device 620 includes two heating elements 608 and 609. Inone embodiment, the first heating element 608 is located on a surface ofthe insulating substrate 602. The first electrode 610 and the secondelectrode 612 are located on a surface of the first heating element 608and spaced from each other. The color element 618 is located on thesurface of the first heating element 608 and between the first electrode610 and the second electrode 612. The second heating element 609 islocated on a surface of the color element 618 and covers the firstelectrode 610 and the second electrode 612. The heating elements 608 and609 are single drawn carbon nanotube film, which is transparent and freestanding. Because the thermochromatic device 620 has two heatingelements 608 and 609, the color element 618 can be heated effectively.Therefore, the thermochromatic device 620 has an improved responsespeed.

Thermochromatic device 620 can be made by the following steps:

-   -   (k) laying a first drawn carbon nanotube film on a surface of        the insulating substrate 602 as a first heating elements 608;    -   (l) applying a first electrode 610 and a second electrode 612 on        a surface of the first heating elements 608;    -   (m) depositing a layer of Ag₂HgI₄ between the first electrode        610 and the second electrode 612 as a color element 618; and    -   (n) laying a second drawn carbon nanotube film on a surface of        the color element 618 to cover the first electrode 610 and the        second electrode 612.

Referring to FIG. 10, a thermochromatic device 720 of one embodimentincludes an insulating substrate 702, a color element 718, a heatingelement 708, a first electrode 710, and a second electrode 712. Thethermochromatic device 720 is similar to the thermochromatic device 220described above except that a recess 722 is formed on a surface of theinsulating substrate 702, and the color element 718 is located in therecess 722. In one embodiment, the color element 718 is located in andfills the recess 722. The heating element 708 is located on a surface ofthe insulating substrate 702 and covers the recess 722. The firstelectrode 710 and the second electrode 712 are located on a surface ofthe heating element 708 and spaced from each other. The heating element708 is a single drawn carbon nanotube film which is a transparent andfree standing film. The color element 718 can remain in a shapesubstantially the same as the shape of the recess 722 during heatingprocess.

Thermochromatic device 720 can be made by the following steps:

-   -   (o) forming a recess 722 on a surface of the insulating        substrate 702;    -   (p) depositing a layer of Ag₂HgI₄ in the recess 722 as a color        element 718;    -   (q) laying a single drawn carbon nanotube film on a surface of        the insulating substrate 702 as the heating element 708, wherein        the heating element 708 covers the recess 722; and    -   (r) applying a first electrode 710 and a second electrode 712 on        a surface of the heating elements 708.

The disclosure further provides a thermochromatic display apparatususing the thermochromatic device described in above embodiments. Thethermochromatic display apparatus includes a plurality ofthermochromatic devices arranged to form a pixel matrix, a drivingcircuit capable of controlling the plurality of thermochromatic devicesand a number of lead wires configured to electrically connect thedriving circuit and the number of thermochromatic devices. The number ofthermochromatic devices can use one common insulating substrate and becontrolled by an addressing circuit. The thermochromatic displayapparatus using the thermochromatic device 220 of the first embodimentis given below to illuminate the thermochromatic display apparatus ofthe disclosure.

Referring to FIGS. 11 and 12, a thermochromatic display apparatus 20includes an insulating substrate 202, a number of substantially parallelfirst electrode down-leads 204, a number of substantially parallelsecond electrode down-leads 206, and a number of thermochromatic devices220. The number of first and second electrode down-leads 204, 206 arelocated on the insulating substrate 202. The first electrode down-leads204 are generally set at an angle to the second electrode down-leads 206forming a grid. A cell 214 is defined by each two substantially adjacentfirst electrode down-leads 204 and each two substantially adjacentsecond electrode down-leads 206 of the grid. One of the thermochromaticdevices 220 is located in each cell 214. Each thermochromatic device 220corresponds to a pixel of the thermochromatic display apparatus 20.

The insulating substrate 202 is configured for supporting the firstelectrode down-leads 204, the second electrode down-leads 206, and thethermochromatic devices 220. The shape, size, and thickness of theinsulating substrate 202 can be chosen according to need. In oneembodiment, the insulating substrate 202 is a square PET substrate witha thickness of 1 millimeter and an edge length of 48 millimeters. Thenumber of thermochromatic devices 220 uses a common insulating substrate202.

The first electrode down-leads 204 are located equidistantly apart. Adistance between adjacent two first electrode down-leads 204 can rangefrom about 50 micrometers to about 2 centimeters. The second electrodedown-leads 206 are located equidistantly apart. A distance betweenadjacent two second electrode down-leads 206 can range from about 50micrometers to about 2 centimeters. A suitable orientation of the firstelectrode down-leads 204 and the second electrode down-leads 206 arethat they be set at an angle with respect to each other. The angle canrange from about 10 degrees to about 90 degrees. In one embodiment, theangle is 90 degrees, and the cell 214 is a square area.

The first electrode down-leads 204 and the second electrode down-leads206 are made of conductive material such as metal or conductive slurry.In one embodiment, the first electrode down-leads 204 and the secondelectrode down-leads 206 are formed by applying conductive slurry on theinsulating substrate 202 using screen printing process. The conductiveslurry composed of metal powder, glass powder, and binder. The metalpowder can be silver powder, the glass powder having low melting point,and the binder can be terpineol or ethyl cellulose (EC). The conductiveslurry can include about 50% to about 90% (by weight) of the metalpowder, about 2% to about 10% (by weight) of the glass powder, and about8% to about 40% (by weight) of the binder. In one embodiment, each ofthe first electrode down-leads 204 and the second electrode down-leads206 is formed with a width in a range from about 30 micrometers to about100 micrometers and with a thickness in a range from about 10micrometers to about 50 micrometers. However, it is noted thatdimensions of each of the first electrode down-leads 204 and the secondelectrode down-leads 206 can vary corresponding to dimension of eachcell 214.

The first electrodes 210 of the thermochromatic devices 220 arranged ina row of the cells 214 can be electrically connected to the firstelectrode down-lead 204. The second electrodes 212 of thethermochromatic devices 220 arranged in a column of the cells 214 can beelectrically connected to the second electrode down-lead 206.

Each of the first electrodes 210 can have a length in a range from about20 micrometers to about 15 millimeters, a width in a range from about 30micrometers to 10 millimeters and a thickness in a range from about 10micrometers to about 500 micrometers. Each of the second electrodes 212has a length in a range from about 20 micrometers to about 15millimeters, a width in a range from about 30 micrometers to about 10millimeters and a thickness in a range from about 10 micrometers toabout 500 micrometers. In one embodiment, the first electrode 210 has alength in a range from about 100 micrometers to about 700 micrometers, awidth in a range from about 50 micrometers to about 500 micrometers anda thickness in a range from about 20 micrometers to about 100micrometers. The second electrode 212 has a length in a range from about100 micrometers to about 700 micrometers, a width in a range from about50 micrometers to about 500 micrometers and a thickness in a range fromabout 20 micrometers to about 100 micrometers.

The first electrodes 210 and the second electrode 212 can be made ofmetal or conductive slurry. In one embodiment, the first electrode 210and the second electrode 212 are formed by screen printing theconductive slurry on the insulating substrate 202. As mentioned above,the conductive slurry forming the first electrode 210 and the secondelectrode 212 is the same as the electrode down-leads 204, 206.

Furthermore, the thermochromatic display apparatus 20 can include aplurality of insulators 216 sandwiched between the first electrodedown-leads 204 and the second electrode down-leads 206 to avoidshort-circuiting. The insulators 216 are located at every intersectionof the first electrode down-leads 204 and the second electrodedown-leads 206 for providing electrical insulation therebetween. In oneembodiment, the insulator 216 is a dielectric insulator.

In one embodiment, 16×16 (16 rows, and 16 thermochromatic devices 220 oneach row) thermochromatic devices 220 are arranged on a square PETinsulating substrate 202 with an edge length of 48 millimeters. Eachheating element 208 is a single drawn carbon nanotube film with a lengthof 300 micrometers and a width of 100 micrometers. The single drawncarbon nanotube film is fixed on the surface of the insulating substrate202 with an adhesive. The ends of the heating element 208 are locatedbetween the insulating substrate 202 and the electrodes 210 and 212. Thecarbon nanotubes of the heating element 208 extend from the firstelectrode 210 to the second electrode 212.

Furthermore, the thermochromatic display apparatus 20 can include aheat-resistant material 222 located around each thermochromatic device220. The heat-resistant material 222 can be located in a space betweenthe thermochromatic device 220 and the electrode down-leads 204, 206 inthe cell 214. The thermochromatic devices 220 in adjacent cells 214 areheat insulated and will not interfere with each other. Theheat-resistant material 222 can be aluminum oxide (Al₂O₃) or organicmaterial such as PET, PC, PE, or PI. In one embodiment, theheat-resistant material 222 is PET with a thickness same as thethickness of the electrode down-leads 204, 206. The heat-resistantmaterial 222 can be formed by printing, chemical vapor deposition (CVD)or physical vapor deposition (PVD).

Furthermore, the thermochromatic display apparatus 20 can include aprotecting layer 224 located on the insulating substrate 202 to coverall the electrode down-leads 204, 206, and the thermochromatic devices220. The protecting layer 224 is an insulating transparent layer thatcan be made of aluminum oxide (Al₂O₃), silicon dioxide (SiO₂), ororganic material such as PET, PC, PE, or PI. The thickness of theprotecting layer 224 can be selected according to need. In oneembodiment, the protecting layer 224 is a PET sheet with a thickness ina range from about 0.5 millimeter to about 2 millimeters. The protectinglayer 224 can prevent the thermochromatic display apparatus 20 frombeing damaged and polluted.

In use, the thermochromatic display apparatus 20 can include a drivingcircuit (not shown) to drive the thermochromatic display apparatus 20 todisplay an image. The driving circuit can control the thermochromaticdevices 220 through the electrode down-leads 204, 206 to display adynamic image. The thermochromatic display apparatus 20 can be used in afield of advertisement billboard, newspaper, or electronic book.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Any elements describedin accordance with any embodiments is understood that they can be usedin addition or substituted in other embodiments. Embodiments can also beused together. Variations may be made to the embodiments withoutdeparting from the spirit of the disclosure. The above-describedembodiments illustrate the scope of the disclosure but do not restrictthe scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A thermochromatic device, comprising: aninsulating substrate having a top surface; a color element, located onthe top surface of the insulating substrate, comprising a reversiblethermochromatic material; a first heating element configured to supplyheat pulses to heat the color element, wherein the first heating elementis a carbon nanotube structure, and the carbon nanotube structure is asubstantially pure structure of carbon nanotubes; and a first electrodeand a second electrode electrically connected to the first heatingelement.
 2. The thermochromatic device of claim 1, wherein the colorelement and the first heating element are stacked on each other.
 3. Thethermochromatic device of claim 2, wherein the first heating element islocated between the insulating substrate and the color element.
 4. Thethermochromatic device of claim 2, wherein the first heating element islocated on the color element and the color element is located betweenthe insulating substrate and the first heating element.
 5. Thethermochromatic device of claim 1, wherein the first heating element isspaced from the color element and is in contact with the first electrodeand the second electrode, and the first heating element and the colorelement are configured to exchange heat primarily by radiation.
 6. Thethermochromatic device of claim 1, wherein the insulating substrate hasa recess defined on the top surface, the color element is located in therecess and covered by the first heating element, the first heatingelement covers the recess, and the first electrode and the secondelectrode are located on the first heating element so that at least partof the first heating element is sandwiched between the insulatingsubstrate and one of the first electrode and the second electrode. 7.The thermochromatic device of claim 1, wherein the reversiblethermochromatic material has a color change temperature below 200° C. 8.The thermochromatic device of claim 1, wherein the reversiblethermochromatic material comprises an inorganic reversiblethermochromatic material; the inorganic reversible thermochromaticmaterial comprises a material that is selected from the group consistingof silver iodide, silver complex, silver double salt, copper iodide,copper complex, copper double salt, mercury iodide, mercury complex,mercury double salt, cobalt salt, nickel salt, methenamine compound,vanadium (III) oxide, chromate, vanadate, and combinations thereof. 9.The thermochromatic device of claim 1, wherein the reversiblethermochromatic material comprises an organic reversible thermochromaticmaterial; the organic reversible thermochromatic material comprises acolor fixative, a color developing agent, and a solvent.
 10. Thethermochromatic device of claim 1, wherein the reversiblethermochromatic material comprises a thermotropic liquid crystal. 11.The thermochromatic device of claim 1, wherein the carbon nanotubestructure comprises at least one carbon nanotube film.
 12. Thethermochromatic device of claim 11, wherein a heat capacity per unitarea of the at least one carbon nanotube film is less than 2×10⁻⁴J/m²·K.
 13. The thermochromatic device of claim 11, wherein the at leastone carbon nanotube film comprises a plurality of carbon nanotubessubstantially oriented along a same direction that extends from thefirst electrode to the second electrode.
 14. The thermochromatic deviceof claim 13, wherein the plurality of carbon nanotubes of the at leastone carbon nanotube film are joined end-to-end by Van der Waalsattractive force therebetween.
 15. The thermochromatic device of claim1, wherein the carbon nanotube structure comprises at least one carbonnanotube wire; and the at least one carbon nanotube wire is a twistedcarbon nanotube wire or untwisted carbon nanotube wire.
 16. Athermochromatic display apparatus, comprising: a plurality ofthermochromatic devices arranged to form a pixel matrix, and each of theplurality of thermochromatic devices comprises: an insulating substratehaving a top surface; a color element, located on the top surface of theinsulating substrate, comprising a reversible thermochromatic material;a heating element configured to supply heat pulses to heat the colorelement, wherein the heating element is a carbon nanotube structure, andthe carbon nanotube structure is a substantially pure structure ofcarbon nanotubes; and a first electrode and a second electrodeelectrically connected to the heating element; a driving circuit forcontrolling the plurality of thermochromatic devices; and a plurality oflead wires configured to electrically connect the driving circuit andthe plurality of thermochromatic devices.
 17. The thermochromaticdisplay apparatus of claim 16, wherein the color element is locatedbetween the insulating substrate and the first heating element.
 18. Thethermochromatic display apparatus of claim 16, wherein the heatingelement is spaced from the color element, and the heating element andthe color element are configured to exchange heat primarily byradiation.
 19. The thermochromatic display apparatus of claim 16,wherein the insulating substrate has a recess defined on the topsurface, and the color element is located in the recess and covered bythe heating element.
 20. A thermochromatic device, comprising: aninsulating substrate comprising a top surface; a color element on thetop surface of the insulating substrate, comprising a reversiblethermochromatic material; a first heating element comprising a carbonnanotube structure and a second heating element, wherein the firstheating element and the second heating element are parallel to eachother, the color element is sandwiched between the first heating elementand the second heating element, and the first heating element and thesecond heating element are configured to supply heat pulses to the colorelement; and a first electrode and a second electrode, each of the firstelectrode and the second electrode being electrically connected to boththe first heating element and the second heating element.